The present invention relates to the use of phase equilibria as shown in the phase diagram of Cu—In—Se for the preparation of solid Cu—In—Se phases with defined compositions. In particular, a new method is provided for obtaining single-phase α-CuInSe2 directly from a liquid phase. Further, the new method allows one to fabricate single phase α-CuInSe2 with compositions that one could not obtain before.
Cu—In—Se-compositions, e.g. compounds and alloys, particularly the α-CuInSe phase having the stoichiometry CuInSe2 are semiconductor materials which are suitable for photovoltaic applications due to their excellent optical and electronical properties. Since CuInSe-compounds transfer light energy into electric energy with high efficiency and additionally absorb visual light very well, these materials are suitable for preparing thin film solar cells, which are superior to conventional silicon solar cells both as to their performance characteristics and lower material consumption and thus can be produced at lower costs. Due to their higher efficiency and weight reduction because of the lower quantity of material present, such solar cells are particularly well suited for the energy supply of space ships and satellites. Theoretically, CuInSe2 thin film solar cells are said to achieve efficiencies of over 20%. The efficiencies obtained in practice, however, are still below 15% so far. An important aspect for the efficiency is the microstructure of the thin films, i.e. the entirety of the microscopic defects in the material concerned. The microstructure comprises point defects (impurities), dislocations, surface and inner boundaries. The inner boundaries comprise grain boundaries, i.e. boundaries between areas having different orientation of the crystal lattice, and phase boundaries.
Among the defects mentioned above, extended defects such as grain boundaries and phase boundaries presumably have a particular disadvantageous effect on the photovoltaic properties of the material: Photovoltaically produced charge carriers, i.e. electrons and holes are captured by these defects and recombine with one another thereby generating a photon. Thus, at crystal defects the photovoltaic conversion of energy is reversed and the macroscopically detectable efficiency of the solar cell decreases accordingly. In addition, the presence of additional phases reduces the volume percent of the photovoltaically relevant α-phase, thereby reducing the efficiency even more.
Accordingly, a particular high efficiency can be achieved by producing an α-copper-indium-selenide without grain boundaries and phase boundaries. Such a monocrystalline material without grain boundaries and phase boundaries is called single crystal.
Over the recent years there have been intensive attempts of producing a single-crystalline α-copper-indium-selenium. However, the Bridgman techniques applied for this purpose have not been successful. According to current knowledge, the problem is that in the Cu—In—Se system a complex variety of phase equilibria can be generated. For instance, cooling down a liquid phase having the composition CuInSe2 to room temperature according to the Bridgman method does not result in a single crystal, but in a mixture having several phases of different composition and corresponding phase boundaries. To be able to conceive a method for the production of single crystals a complete and precise ternary phase diagram is required. So far, however, the literature only provides incomplete and/or incorrect phase diagrams of the Cu—In—Se system.
U.S. Pat. No. 4,652,332 describes a method, wherein stoichiometric amounts of Cu, In and Se are applied. Contrary thereto, the method according to the invention does not start out from the stoichiometric composition. Additionally, the phase diagram included in U.S. Pat. No. 4,652,332 differs from the phase diagram presented herein.
L. S. Yip et al., Record of the Photovoltaic Specialists Conference, U.S., New York, IEEE, vol. Conf. 21, May 21, 1990, pages 768–771 show in FIG. 3 compositions with which α-CIS-monocrystals were obtained. Upon performing the works of the present invention it was noticed that the breadth of the α phase is much smaller and the monocrystals described in Yip et al. are no thermodynamically stable α-CIS single crystals. Yip et al. mention data, which were obtained by electron probe microanalysis (EPMA). The spacial resolution of this method, however, is only 1 μm (1000 nanometers). When corresponding compositions are examined with transmission electron microscopy, a technique used for obtaining a spacial resolution of 0.3 nm, however, polyphases are detected. The monocrystals described by Yip et al. are thus polyphase crystals having a common crystal orientation, contrary to the single-phase crystals described herein. Further, the crystals described by Yip et al. have a size of only a few millimeters, whereas the method described herein allows for the production of crystals of arbitrary size. Moreover, the Bridgman method of Yip et al. is based on congruent solidification, which means that the solidifying solid particle has the same composition as the melt. A particuliarity of the method according to the invention is that the melt from which the α-CIS single-phase crystals are obtained exhibits a different composition than, α-CIS.
Z. A. Shukri et al., Journal of Crystal Growth, NL, North-Holland Publishing, Amsterdam, vol. 191, No. 1–2, (1998), pages 97–107 describe a Bridgman method based on concruent solidification. This means that the solidifying solid particle has the same composition as the melt. Contrary thereto the melt, from which α-CIS monocrystals are obtaned in the method according to the invention, exhibits a different composition than α-CIS. Also Shukri et al. show measuring data obtained by electron probe microanalysis (EPMA). The dimensional resolution of this method, however, is only 1 μm (1000 nm). When crystals of corresponding compositions are examined with transmission electron microscopy, a technique for obtaining a dimensional resolution of 0.3 nm, the polyphases of such crystals can be detected.
Abid et al., Conference Record of the 19th Photovoltaic Specialists Conference 1987, May 1987, pages 1305–1308 also describe a phase diagram of Cu—In—Se, which is incomplete and different to the phase diagram presented herein. The assertion in Abid et al. that monocrystals of the alloy concerned were produced is not supported by experimental data in this work.
In extensive experimental tests conducted on over 250 different Cu—In—Se alloys for the first time the ternary phase diagram of the Cu—In—Se system was determined completely and at high accuracy with the help of differential-thermal analysis, transmission electron microscopy, scanning electron microscopy, x-ray diffraction and light microscopy. From these phase diagrams now several promising methods for the production of α-single crystals and other interesting compositions can be derived.
From the ternary phase diagram Cu—In—Se, as shown in the enclosed figures, it becomes evident why α-single crystals cannot be produced according to the Bridgman method. This phase diagram shows that the α-phase is involved in a variety of phase equilibria at temperatures during the cooling phase to room temperature. At about 500° C. there are two-phase equilibria and three-phase equilibria between the α-phase and nine other phases. However, this was not known before our tests have been performed. According to the prior art it was assumed that α-CuInSe2 can only be generated via solid phase transformation δH→α. According to our novel ternary phase diagram the α-phase, however, has four different surface areas of primary crystallization—consequently, there are four types of liquid phases, from which CuInSe2 can be directly obtained and then cooled down to room temperature without further phase transformation.
Thus, the novel phase diagram of Cu—In—Se including the liquidus surface of the ternary system as described in the accompanying figures provides novel methods for the preparation of solid compositions comprising the elements Cu, In and Se. By means of the phase equilibria in the diagram the direct formation of desired solid compositions, i.e. solid compositions having a desired phase and/or stoichiometry, may be effected from a liquid phase without phase transformation in the solid state. Preferably, the solid compositions are prepared by crystallization from a liquid molten Cu—In—Se phase.
For example, the preparation of the solid compositions may comprise crystal growth by the Czochralski method without feed of a liquid phase. In this method, a seed crystal is immersed in a melt having a desired composition and temperature and grown by rotation. In a further embodiment, the preparation of the solid composition comprises crystal growth by the Czochralski method including feed of a liquid phase, i.e. during the crystal growth a liquid phase having the same composition or a different composition from the original liquid phase is fed continuously or intermittently into the liquid phase which is used for the crystal growth.
Alternatively, the preparation of single crystals may comprise crystal growth from a first liquid phase which is in contact with a second liquid phase wherein the density and the stoichiometry of the first liquid phase differs from the density and the stoichiometry of the second liquid phase. While the crystal is grown from the first liquid phase, the second liquid phase supplies the material withdrawn from the first liquid phase by the crystal growth.
It should be noted, however, that other methods for crystal growth, particularly for the growth of single crystals are suitable for the preparation of particular Cu—In—Se-compositions as can be gathered by the skilled person from the phase diagram as shown in the accompanying figures. For a detailed description of crystallization methods reference is made to Elwell and Scheel, Crystal Growth from High Temperature Solutions, New York: Academic Press (1975), Holden and Morrison, Crystals and Crystal Growing: The MIT Press (1982), Recker and Wallrafen, Synthese, Einkristallzüchtung und Untersuchung akustooptischer Materialien FbNRW, 2983 (1980), Wilke, Kristallzüchtung (Ed. J. Bohm), Berlin: VEB Deutscher Verlag der Wissenschaften (1988) and references recited therein.
A particular advantage of the novel Cu—In—Se phase diagram including the liquidus surface of the system is to provide novel methods for preparing compositions from a liquid phase wherein the desired solid composition has a stoichiometry which differs from the stoichiometry of the liquid phase. Without knowledge of the liquidus surface of the system such a method of preparation would not be feasible.
The phase diagram is particulary suitable for the preparation of alloys that are single phase compositions and/or single crystalline compositions. The preferred direct formation of the desired solid alloy from a liquid phase without solid state transformation results in a composition having advantageous properties compared to prior art compositions which have been subjected to solid state transformations. Alloys obtained by direct formation from a liquid phase have advantageous physicochemical properties compared to alloys that have been obtained via a solid state transformation, e.g. macroscopic homogeneity, lower amounts of defects and inner boundaries. Particularly, alloys obtained by direct precipitation from a liquid phase will not contain any inter-phase interfaces, which also means that they will be free of the internal stresses that usually build up in the presence of such interfaces.
The phase diagram as shown in the accompanying figures is not restricted to a ternary system consisting of the elements Cu, In and Se, but may be extended to Cu—In—Se alloys doped with at least one further element, particularly Ga, Na, or S, wherein said at least one further element is present with dopant concentrations up to 5 atom percent, preferably up to 2 atom percent and more preferably up to 1 atom percent. The before-mentioned dopants widen the α-phase field, the stoichiometrical range of the α-phase.
The phase diagram of the present invention allows the preparation of single phase and/or single crystalline alloys in the ternary system Cu, In and Se and extensions thereof comprising at least one further element. More preferably, the compositions are selected from the α-phase, the γT-phase, the δR-phase and the δH-phase. The melt compositions and temperatures for a direct deposition of γT, δR and δH from a melt are indicated in FIGS. 6, 43(a), 50 and 51. Most preferably, the composition is the α-phase having the stoichiometry CuInSe2 within a compositional range as indicated in FIGS. 51 and 52. Doping with Ga, Na or S further extends the compositional range of the α-phase.
The α-phase may be directly crystallized from a liquid phase under conditions selected from the group of liquidus surfaces of primary crystallization αL1, αL2, αL3 and αL4 as defined in the phase diagram and particularly in Table 4.
Thus, a further subject matter of the invention is a method for directly obtaining the α-phase from a liquid phase by crystal growth under conditions selected from the group of liquidus surfaces of primary crystallization αL1, αL2, αL3 and αL4. The α-phase may be obtained as a single phase composition, and thus also as a single crystal. Further, it is possible to grow single crystalline α epitaxially on a single crystalline substrate, such as a metal or ceramic substrate.
Yet a further subject matter of the present invention is a single phas composition, selected from the α-phase, the γT-phase, the δR-phase and the δH-phase, particularly which have been directly obtained from a liquid phase and which have not been subjected to a solid phase transformation. Single crystals of this kind can have a defect concentration as low as the defect concentration in melt-grown single crystals of any other comparable material. Since the solidification of the α-phase from the liquid phase L1, L2, L3 or L4 can be conducted close to thermodynamic equilibrium (small supercooling), the defect density of the resulting crystals will be very low compared to, for example, material deposited from the vapor phase (physical vapor deposition, PVD) or layers plated from aqueous solutions. Similar to Si crystals grown from the melt, the concentration of extended defects (phase boundaries, grain boundaries, twin boundaries, stacking faults and dislocations), which one can experimentally observe by transmission electron microscopy (TEM), can be near or equal to zero. For the same reason, the concentration of various types of point defect, which one can analyze by capacity methods (admittance spectroscopy, deep level transient spectroscopy) can be as low as the corresponding equilibrium concentration Exp[S/kBT]·Exp[−E/kBT], where S denotes the formation entropy of the respective type of point defect, E the formation energy, kB Boltzmann's constant, and T the temperature in Kelvin. Further, the composition may be a macroscopic single crystal having a size of at least 1 cm3, preferably at least 5 cm3.
Particularly preferred methods for obtaining α-CuInSe2 single crystals are described in detail as follows:
The novel phase diagram shows that α-single crystals with a certain variety of compositions can be generated around the ideal stochiometric composition CuInSe2 (FIGS. 43a–g). This does not work with the Bridgman technique, either: Since the Bridgman technique is bound to congruent solidification of the liquid phase, the single crystals of the δ-phase are always obtained first. As can be gathered from the novel phase diagram, the δ-phas crystallizes via a melting point maximum at 1002° C. thus resulting in the composition Cu23.5In26.0Se50.5. Since this composition differs from the ideal composition Cu25In25Se50 of the α-phase, the high temperature phase δ does not completely transform into the α-phase upon further cooling, but decomposes into two phases, α and δR. This can be taken from the quasibinary section In2Se3—Cu2Se (FIGS. 6a–d) of the ternary phase diagram.
Based on this novel ternary phase diagram Cu—In—Se, there are two preferred methods for the production of α-single crystals:    1. Czochralski method (including and without feed of a liquid phase)    2. Single crystal growth in a monotectic reaction
For performing these methods the location of the composition ranges of the four above-mentioned liquidus surfaces of primary crystallization of a and their temperature dependency as well as position and temperature of the monotectic reaction must be known. According to the polythermic illustration in FIGS. 43a–g the four surfaces of primary crystallization of the α-phase are specified by the parameters indicated in Table 4.
In order to be able to precisely adjust the composition of the single crystals, the course of the tie lines between the α-phase and the liquid phases L1–L4 is provided. Our results prove that the In2Se3—Cu2Se isopleth constitutes a quasibinary section. In quasibinary sections the tie lines are aligned with the section plane. This is an advantage for single crystal growth according to the Czochralski method, since the isopleths immediately disclose the temperature-concentration-range suitable for crystal cultivation. Further, the chemical composition of the liquid phase and the single crystal may be determined and/or adjusted via the tie lines.
This is now possible due to our results, particularly with the isopleth CuInSe2—Cu50In50 (FIG. 32) and—correspondingly—with the isopleth CuInSe2—Cu70In30 (FIG. 31).
Thus, there are at least three novel methods for producing single crystals from α-copper-indium-selenide using isopleth CuInSe2—Cu50In50 (FIG. 32):    1. The Czochralski method without feed of a liquid phase provides a means for growing the α-phase as single crystal starting from a composition with 41 atom percent (at. %) Se to the monotectic point    2. The Czochralski method including feed of a liquid phase provides a means for producing single crystals within the concentration range from 33.7 at. % Se to 41 at. % Se at any temperature between 812° C. and 660° C., as the tie lines are aligned with the section plane.    3. Furthermore, the isopleth CuInSe2—C50In50 shows a monotectic reaction, wherein the liquid phase L1 dissolves at 600° C. into the α-phase and another liquid phase L2. This liquid phase L2 is richer in indium than liquid phase L1 and has therefore a higher density (a higher specific weight). Due to gravitation, the liquid phases L1 and L2 are positioned in layers one on top of the other and whereas the single crystal is grown from liquid phase L1, the liquid phase L2 below supplies the liquid phase L1 with further material.
The isopleth CuInSe2—Cu70In30 (FIG. 31) provides further possibilities for the production of α-single crystals. This isopleth can be used for producing single crystals of the α-phase according to the three above-mentioned methods between 33.5 at. % Se at 673° C. and 39.6 at. % Se at 805° C.
Moreover, the isopleth InSe—CuSe (FIGS. 30 and 51) enables the growth of single crystals of the α-phase from the liquid phase L3 according to both Czchoralski methods. The following data have to taken into consideration: Between 44 at. % Se at 800° C. and 49.5 at. % Se at 605° C. the tie lines are aligned with the section plane and form the boundary between the α-phase and the liquid phase L3.
The same is true for isopleth Se—CuInSe2 (FIG. 45). Between 85 at. % Se at 805° C. and 99.99 at. % at 221° C. the α-phase can be primarily grown from the liquid phase L4. Hence the preconditions for single crystal growth of α-copper-indium-selenide without phase transformation are fulfilled.
The invention is further explained by the following figures, tables and examples: